1. Field of the Invention
This invention relates generally to a material and the process for depositing coatings with superior dielectric properties to be used as dielectrics in rolled or parallel plate capacitors. More particularly, the invention relates to a process for applying coatings to films and foils or parallel plates which can be energized to form capacitors for energy storage.
2. Description of Related Art
Rolled capacitors are typically formed by alternating layers of metal (the electrodes) with layers of polymer dielectrics in a rolled geometry. Parallel-plate capacitors can be manufactured in the same way by alternating layers of rigid electrodes and polymer or ceramic dielectrics. Polymer dielectrics have limitations in temperature stability, dielectric strength, and dissipation factors. For a given voltage rating, the required polymer thickness can be significant, typically greater than 2 micron. Ceramic dielectrics often suffer from low dielectric strengths.
Existing methods for using coatings on metal electrodes as the dielectrics in rolled and parallel-plate capacitors include, e.g., traditional diamond-like carbon films (DLC's), which typically comprise amorphous carbon films which may or may not contain hydrogen, and typically exhibit a mixture of sp3 (diamond-type) and sp2 (graphite-type) chemical bonds, which have been proposed as a dielectric coating material. The resultant deposition rates are often very slow, and hence, of no commercial value for this application.
Plasma deposition methods for the application of dielectric coatings suffer from one or more of the following deficiencies and shortcomings:
(1) Difficulty in pre-cleaning of substrates prior to deposition;
(2) Poor adhesion of the dielectric layer;
(3) High permeation of the coatings by water vapor and oxygen and subsequent degradation of dielectric properties;
(4) Fabrication of defect-laden or permeable, low density coatings;
(5) Poor control of coating properties during a deposition run;
(6) Poor coating thickness control and reproducibility of thickness;
(7) Coating stress is sufficient to cause wrinkling of the foil or film substrate or electrode;
(8) Poor control of coating uniformity along a long length of substrate;
(9) Inability to scale-up the deposition process for mass production; and
(10) Brittle coatings which crack or craze during rolling processes.
Ion beam etching and deposition of many materials, including webs and foils is known. Surface modification processes or thin film deposition processes by ion beams are routinely used for food packaging to decrease permeability to oxygen and lengthen shelf lives of products.
Unfortunately, due to the high compressive stress it is difficult to deposit traditional DLCs on soft plastics and films and foils such as polycarbonate or polyester to thicknesses greater than 0.1 microns without the formation of stress cracks or wrinkling of the substrate. DLC is therefore unsuitable as a thick (i.e. greater than 0.1 micron thick coating) on such substrates.
The following documents illustrate existing coating processes and dielectric coatings:
Kaganowicz, U.S. Pat. No. 4,168,330, describes a process for depositing a silicon dioxide layer on a substrate by activating a mixture of cyclic siloxanes and oxygen “around the substrate by means of a glow discharge.” It is taught that this plasma polymerization process was designed for depositing thin dielectric layers on audio/video discs.
S. Fries-Carr, R. L. C. Wu and P. B. Kosel, U.S. Pat. No. 5,844,770, describes the use of dielectric coated materials in rolled capacitors
B. Knapp, F. Kimock, R. Petrmichl, N. Galvin, U.S. Pat. No. RE37294 describe the deposition of abrasion-resistant coatings using ion beam deposition processes.
Petrmichl, R., Knapp, B., Kimock, F., Daniels, B., U.S. Pat. No. 5,618,619 describes the material and composition of a Highly Abrasion Resistant Flexible Coatings for Soft Substrates.
However, none of these documents disclose a method for producing a dielectric coated substrate that has all of the properties necessary or desirable for incorporation into a rolled capacitor, including high dielectric constant, high dielectric strength, high breakdown voltage, low dissipation factor, and is highly adherent and able to be rolled. Thus, there remains a need in the art for a method for producing such a dielectric-coated substrate, as well as for the substrate itself and capacitors produced therefrom.